It is well known that drying of milk concentrates and similar liquids may be performed in two steps, viz a first step, in which the liquid is atomized in a stream of hot air in a drying chamber to produce moist particles which in a second step are further dried to the desired moisture content in a fluidized bed, cf. U.S. Pat. No. 3,956,521. Said splitting up of the drying process in two steps involves certain advantages as to heat economy and product quality over conventional one step spray drying processes in which no drying in fluidized bed is used.
From French patent specification No. 1,526,478 and from published German patent application No. 2,125,155 it is known to arrange a fluidized bed in the bottom part of the drying chamber of a spray drying or spray cooling unit. Said use of spray drying or cooling over a layer of fluidized particles produced by the spray drying or cooling is described as suitable for producing relatively large granules of the desired products, which primarily are fertilizers.
When producing fertilizers it is preferred to achieve monogranular granules, which term in the present specification and in the attached claims means granules having a very compact structure with little interstitial air due to the fact that they have an onion skin-like structure or consist of particles which are substantially completely fused or coalesced. Such granules have high mechanical strength and dissolve relatively slowly in water, which may be an advantage as far as fertilizers are concerned. Said compact structure is obtained partly as a result of the fact that the droplets to be dried contain a relatively high proportion of dissolved salts.
However, when milk products of the above defined type are concerned, it is desired to obtain the product as agglomerates and not as monogranular granules. The term agglomerates in this context are used as covering clusters of particles which adhere together at certain points of contact, which clusters have a non-compact, porous structure including a substantial amount of interstitial air. When such agglomerated clusters of milk products are introduced in water, they disintegrate and the individual particles forming the clusters are easily dispersed.
It is to be observed that combinations of spray drying and drying or cooling in fluidized bed or fluidized, bed-like systems arranged in one unit have been suggested not only in connection with the production fertilizers.
Thus, French patent specification No. 1,423,070 discloses a spray drier having a fluidized bed in the bottom part. In said specification it is not indicated which type of products is to be produced. However, since the apparatus disclosed in said French specification is designed to exclude substantial contact between the fluidized layer and the particles in the spray drying zone, it is unsuitable for obtaining the purpose of the present invention, as it will appear from the below explanation.
Published German patent application No. 2,122,237 discloses a method for drying of food products or chemical or pharmaceutical products among which are mentioned milk products such as yogurt and quark. By said method a spray drying is combined with a drying in fluidized bed. It is not defined whether these two treatments are carried out in two different apparatuses or in one apparatus. The method comprises a first step, in which the particles to be dried move counter-current to the drying air, and a second step wherein the particles receive a final drying in a fluidized bed, whereby agglomerates are created. To reduce heat damage, relatively low temperatures are used for the drying air, which necessitates a previous dehumidification thereof. Use of a counter-current principle in the first drying step involves that the method results in a rather extensive heat damage of the product relative to the damage involved in co-current processes. For this reason it is not possible to use as high an inlet temperature of the drying air in first step as it would be desired for obtaining low heat consumption. Moreover, the application of the counter-current principle only permits an extremely limited velocity for the drying air in the first drying step, which in turn involves that the apparatus required for obtaining a certain drying capacity necessarily has to be rather voluminous and consequently the investments become high. Besides it is often desired to achieve a more extensive agglomeration than the one which is obtained in a conventional fluidized bed as the one which is used in said published application.
In the paper J. Due Jensen: "Agglomerating, Instantizing, and Spray Drying" (Food Technology, vol. 29, no. 6, June 1975, pages 60-71) a drying operation is disclosed, in which the liquid to be dried is atomized over or into a fluidized bed, and it is stated that in this way particle growth can occur either as agglomeration of several particles, or as layering of solid onto individual particles to obtain the so called onionskinning effect.
However, said method is of another type than the process according to the present invention, because the total drying takes place in the very fluidized layer, which involves drawbacks because of risk of heat damage. Moreover the drying capacity is rather limited.
French patent specification no. 2,095,614 discloses spray drying of milk in a spray drying tower in which a layer, described as a fluidized layer, is maintained in the bottom part. However, it appears that said layer is not a proper fluidized layer (viz a layer of particles acting as a liquid) and it is indicated as having a thickness of only 2-3 mm, which would in no way make possible the achievements of the objects of the present invention, because the layer only acts as a powder slide conveyer.
Also U.S. Pat. No. 3,735,792 discloses a spray drying chamber having means for maintaining a fluidized layer at the bottom part thereof. According to said specification a very specific swirling flow of particles from the fluidized layer to the top section of the drying chamber and back to the fluidized layer is mandatory to achieve the desired results. With that design the atomized droplets of liquid to be dried are contacted with already dried particles, while the droplets are still in liquid state, for which reason the particles produced to a substantial extent will obtain a monogranular structure and not be agglomerated clusters as defined above.